Selected species of rumen bacteria will be studied, singly and in combination, to determine the influence of species interactions on fermentation patterns and growth yields. These studies will be specifically oriented to test the hypothesis that the organic fermentation products of carbohydrate-fermenting organisms which produce hydrogen will be altered significantly in the presence of methanogenic bacteria which produce methane from hydrogen and carbon dioxide. A theoretical basis is presented in the proposal for the suggestion that the alteration in fermentation products can be accompanied by an increase in the ATP produced by the carbohydrate-fermenting species per mole of carbohydrate metabolized. This hypothesis will be tested by growth yield studies. Since several important rumen bacteria produce succinate as a major fermentation end-product, and succinate does not accumulate in the rumen, the possibility that two species can interact to produce propionate through succinate will be tested. Studies with selected species, singly and in combination, will be used to test this possibility. Continuous culture systems will mainly be used. These studies will provide the background for attempts to integrate a minimum number of species to obtain the same overall fermentation found in several important anaerobic microbial ecosystems. These model ecosystem studies will mainly be oriented to reproducing the rumen, anaerobic sewage digestion and large intestine and caecum fermentations. These anaerobic ecosystems are analagous to semi-continuous cultures.